tox: Pass HOME environment variable

Since version 2.0 it only passes a limited set of variables and we need HOME for the tests

tox.ini

@@ -4,6 +4,8 @@ envlist = py26,py27,py33,py34
 deps =
    nose
    coverage
+# We need a valid $HOME for test_compat_expanduser
+passenv = HOME
 defaultargs = test --exclude test_download.py --exclude test_age_restriction.py
     --exclude test_subtitles.py --exclude test_write_annotations.py
     --exclude test_youtube_lists.py